I have a few I will use depending on my mood and the person.
For kids, strike the superman pose and say in a deep loud voice - That is my exclamation mark, I am Emphatic Man!
for others:
I cut myself shaving.
Pretend you have no idea what they are talking about then look down at your chest and go what the ... where did that come from?
and of course the story - I was in the city and saw a woman getting attacked, I ran over to help and the guy pulled a gun, I tried to wrestle it from him but it went off, that little hole is where the bullet went in and that long one is where they had to open me up to get the bullet out.
For some reason no one ever believes that one.
I spend lots of time at the pool, both kids are on a swim team that goes year round, I have found most people will not even notice and some will have this look of horror and disgust when the see the scar, it's usually more fun to be looking them in the face when they look up from the scar, the momentary look of panic, then confusion and the quick oh, someone is calling me act are hysterical.
Whatever you do have fun with it.